Recently, we discovered a cytosolic TMD recognition complex (TRC) that selectively interacts with TA proteins destined for the ER membrane. A central component of TRC was identified as a highly conserved 40 kD ATPase and represents the first molecular factor in this widely used membrane protein insertion pathway (2). Other components that collaborate with TRC40 to mediate selective recognition, targeting, and insertion of TA proteins into the ER are unknown. We are now taking various approaches to identify these additional factor(s) and reconstitute the targeting reaction for TA proteins with defined components. Achieving these goals will define the core machinery and functions for a fundamental protein trafficking pathway and pave the way for future mechanistic and structural analyses.